Previous work (some unpublished) has shown that in frog striated muscle a mechanical coupling between sarcolemma and myofibrils exists. A study will be made of myofiber ultrastructure especially at points where special stress or strain of this coupling can be expected in an attempt to find an anatomical basis for it. Areas studied will include highly stretched fiber tips and the points of attachments of the festoons of the sarcolemma in shortened fibers. Cell structure becomes disorganized in tenotomized frog semitendinosus muscle. Changes from the normal in physiological responses and in ultrastructure will both be studied in the same fiber in preparations of single fibers teased from them. Highly stretched myofibers have non-contractile segments in series with contractile ones, it may also happen in pathological muscle where the sets of myofilaments are disrupted. Interaction of fiber components under these conditions will be studied.